1. Field of the Invention
The present invention relates to the field d personal computing, and in particular, to personal computer modem and I/O technology.
2. Background Art
Input/output (I/O) devices designed for personal computers (PCs) are constrained in their performance by the host PC's I/O standards, as well as physical limitations such as power supply and area allowances. For external devices, these physical limitations can be overcome by dedicated power supplies and larger device housings. For internal devices, those intended to reside inside the host PC housing in expansion card slots, etc., ways must be found to optimize the use of available PC resources. An example of this type of device is the internal modem used to interface the PC with other computer resources over standard phone lines.
As an audible cue to the user, voice band modems are designed to output analog call progress tones from the telephone line to a speaker. In an external modem, this speaker is often a dedicated portion of the modem hardware. For internal PC modems, an alternative to adding a speaker to the I/O card is to use the PC's internal speaker. This is especially important for modems in the PCMCIA environment where there is not room or power for amplifiers and speakers.
Most IBM PC compatibles and laptops provide the ability for I/O cards to drive the internal PC speaker directly. The I/O card interface is limited to a one-bit wide digital output for this purpose, and as such will typically use some form of zero crossing detector to convert the analog signal to a digital representation. This single bit quantization of the analog audio signal creates sharp transitions in the voice signal, effectively adding high frequency components to the voice signal. In addition, the voltage swing is maximized for the digital signal, such that even small analog signals are mapped into the full voltage range. Signal power is therefore relatively fixed near the upper limit.
The physical characteristics of the speaker provide for a frequency response similar to an integrator. The speaker's response, combined with some low pass filtering of the PC itself, removes much of the unwanted high frequency error introduced by the quantization process. The resulting audio signal is therefore of tolerable listening quality. However, the signal power, and therefore its volume, are fixed.
FIG. 1A shows a block diagram of the audio signal path beginning in the telephone line and ending at the PC internal speaker. The two-wire telephone line, comprised of "tip" line 104 and "ring" line 105, is coupled to telephone line interface 100. Telephone interface signal 100 provides analog audio signal 106 to digital comparator 101. Digital comparator 101 provides digital audio signal 107 to low pass filter block 102. Low pass filter 102 provides filtered audio signal 108 to speaker 103.
The circuit of FIG. 1A performs as follows. Typically, telephone interface 100 receives the twisted-pair differential line signal, having both transmit and receive signals, and converts it into two separate differential signals, one for the receive channel and one for the transmit channel. Additionally, the signal on lines 104 and 105 is subject to DC offset. The interface serves to place the analog signal within the limits of the system power supplies, providing a referenced signal. This referenced signal is then provided as signal 106 to digital comparator 101.
Digital comparator 101 outputs a digital "high" value, representing "1", when the differential signal is positive, i.e., the positive line is at a higher voltage level than the negative line. Comparator 101 outputs a digital "low" value, representing "0", when the differential signal is negative. This binary signal is passed as digital audio signal 107 to the PC through the card interface. Low pass filter 102 models the low pass characteristics of the PC and the speaker. The filter acts to soften the fast transitions of the digital signal, providing an output signal 108 with frequency characteristics similar to that of the original audio signal 106. However, the signal power is fixed at the maximum level for the system due to the saturating effect of the quantization process. This loud signal with tolerable sound quality is broadcast by speaker 103 to the user.
The electroacoustic transducer in the PC speaker may be of the electrodynamic, piezoelectric, or electrostatic type. They each have unique mechanical and electrical properties that can cause them to respond differently to the digital signal. It is thus desirable to be able to adjust the volume of the audio signal to compensate for the transducer variability, room acoustics and listener preferences. Previous PC I/O card applications using the PC's internal speaker have not provided the ability to control the loudness of the audio output.